A tubular measured-value sensor for a magnetic-inductive flowmeters can be attached between the connecting flanges of a duct and traversed by a conductive liquid in a direction transverse to the direction of the magnetic field. The voltage proportional to the flow rate is tapped off at two measuring electrodes and conducted via a current conductor to a signal converter. As a rule, the measuring tube of the measured-value sensor consists of a metal tube provided with an inner jacket with an insulating layer or a tube of synthetic material. Mostly, metallic measuring electrodes are used which are inserted in the metal tube and insulated. According to the DE-C2-29 50 039 electrodes can be made of powder- or fiber-like particles of an electrically conductive material, such as carbon, graphite or metal, which are imbedded in an electrically insulating coating in the area of the electrodes.
Further, a measured-value sensor with a ceramic measuring tube as disclosed in DE-A1-23 30 593 which can be fastened between the two flanges of a pipe by interposing sealing rings. In the case of measuring tubes manufactured according to the process of producing ceramic electroinsulators, the electrodes are fused to the vitrified inner jacket and are provided with wire conductors passing through radial bores.
In order to obtain an especially tightly sealed and resistant ceramic measuring tube capable of withstanding liquids under high pressure, and aggressive and hot liquids, in the prior PCT/EP82/00097 it has been proposed to manufacture the measuring tube of vitrified ceramic material into which the shaft of the metallic measuring electrodes is sealingly sintered. Differences in the heat expansion of the ceramic material and the metallic measuring electrodes can, however, lead to stresses and breakage during cooling. The embedding of the electricaly conductive particles in the insulating coating of the measuring tube known from DE-C2-29 50 039 can not be applied to the ceramic measuring tube due to the high sintering temperatures of the ceramic material.
It is also known for measured-value sensors to mount grounding rings between the measuring tube and the connection flanges of the pipe, which are in contact with the conductive liquid and are directly grounded or conductively connected with the grounded metal casing of the measured-value sensor. These grounding rings require sealing rings on both their sides. Such a construction is cumbersome and frequently leads to leakage. An additional operation is also required to form the conductive paths after sintering of the ceramic measuring tube in order to make the grounding rings. In this case a putty with current-conductive particles is applied and the ceramic measuring tube is subsequently reheated to create on the surface of the measuring tube a thin conductive film solidly adhering thereto (compare PCT/EP 82/00097).